Selenous acid oxidant

SYNONYMS Sodium selenite disodium selenite, disodium selenium trioxide, selenious acid, sodium selenium oxide. Sodium selenate disodium selenate Selenium dioxide selenious anhydride, selenium oxide, selenous acid anhydride. 

Synonyms Selenious anhydride Selenium (IV) dioxide (1 2) Selenium oxide Selenous acid anhydride... 

Selenic acid, H2Se04. Formed by oxidation of selenates(IV) (CI2, Mn04 ). A strong acid similar to H2SO4 but loses O2 on heating. 

Oxidative rearrangement of 5a-cholestan-3-one (62) with hydrogen peroxide and a catalytic amount of selenic acid affords 2a-carboxy-A-nor-5a-cholestane, isolated in about 35 % yield as the methyl ester (63)." However, the reaction gives a complex mixture of A-nor- and seco-acids, and under... 

Selenous acid, 0=Se(0H)2, i.e. H2Se03, and tellurous acid, H2Te03, are white solids which can readily be dehydrated to the dioxide (e.g. in a stream of dry air). H2Se03 is best prepared by slow crystallization of an aqueous solution of Se02 or by oxidation of powdered Se with dilute nitric acid ... 

X 10 . Selenates resemble sulfates and both acids form a series of alums (p. 76). Selenic acid differs from H2SO4, however, in being a strong oxidizing agent this is perhaps most dramatically shown by its ability to dissolve not only Ag (as does H2SO4) but also Au, Pd (and even Pt in the presence of Cl ) ... 

Selenic acid, H2Se04, is a strong acid (/fai 2) and the solubility of its salts parallels that of the corresponding sulfates. It is formed by the oxidation of selenous acid or elemental selenium with strong oxidizing agents in the presence of water. Telluric acid, H2Te04, or tellurates are obtained by oxidation of tellurides,... 

Let us add here that despite the general similarities of selenium and sulfur in their chemical properties, the chemistry of selenium differs from that of sulfur in two important aspects their oxoanions are not similarly reduced, and their hydrides have different acid strengths. For example, Se(-HlV) tends to undergo reduction to Se(-II), whereas S(-hIV) tends to undergo oxidation. This difference is evidenced by the ability of selenous acid to oxidize sulftirous acid ... 

Selenium is stable in water and in aqueous solutions over the entire pH interval in the absence of any oxidizing or reducing agent. Selenium can be electrochemically reduced to hydrogen selenide or to selenides that are unstable in water and aqueous solutions. It can be oxidized to selenous acid or selenites and further (electrolyti-cally) to perselenic acid (H2Se20s). Selenic and selenous acids and their salts are stable in water. The selenides, selenites, and selenates of metals other than the alkali metals are generally insoluble. 

The oxidation is regarded as taking place by an electrophilic attack of selenium dioxide (or selenous acid, H2Se03, the hydrate) on the enol of the ketone or aldehyde. This is followed by hydrolytic elimination of the selenium.258... 

Although selenic and telluric acids contain the central atom in the +6 oxidation state, they are very different. The properties of selenic acid, H2Se04, are very similar to those of H2S04, and many of their salts are similar. The oxyacid that contains Te in the +6 state is HsTeOs which can also be written as Te(OH)s. This acid can be prepared from Te or Te02 by suitable oxidation reactions, and it can be also obtained as a solid hydrate. As expected from the formula, telluric acid is a weak acid, although some salts can be obtained in which one or more protons are replaced. 

More recently, the direct synthesis of methanol from methane, using metallic gold as catalyst, was reported, involving a purported CH3-Au intermediate. Selenic acid was used as a stoichiometric oxidant as it is known to oxidize gold metal. Moderate turnovers (30) were achieved (Equation (6)).14... 

Formate esters behave as typical carbonyl compounds in reactions with a number of ylides, eliminating phosphine oxide and forming vinyl ethers, e.g. (33).35 Stabilized phosphoranes are able to condense with the carbonyl group of cyclic thioanhydrides (34).38 Quinoline derivatives, e.g. (35), are obtained from the condensation of dicar-boalkoxy-ylides with isocyanates.37 Benzoyl isothiocyanates and keto-phosphoranes give quantitative yields of (36), which are unreactive in Wittig reactions but can be readily oxidized by selenous acid.38 The products obtained from reactions (Scheme 9) with the triazolinedione (37) depend upon the stability of the ylide used.39... 

Selenic Acid, HaSe04.—This acid was first prepared by Mitscher-lich in 1827, who suspended lead selenate in water and precipitated the lead by means of hydrogen sulphide. By the oxidation of aqueous solutions of selenious acid with suitable reagents, e.g. chlorine or bromine, selenic acid may also be obtained.6 When chlorine is used, any hydrogen chloride must be removed as soon as it is formed, for selenic acid is reduced by hot hydrogen chloride to selenious acid, with liberation of chlorine the process is best carried out6 by passing a... 

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